Thyroid hormones in human milk and their influence on thyroid function of breast-fed babies

Various assay methods for detection of thyroid hormones in human milk were evaluated in recovery and dilution experiments after which the concentrations of thyroxine (T4) and 3,5,3'-triiodothyronine (T3) were measured and compared with those in serum. The effect of breast feeding on pituitary thyroid function of normal babies also was studied. Competitive protein-binding analysis (CPBA) was found to be unsuitable for measurement of T4 in milk. T4 was not detected in samples of human milk by four radioimmunoassays (RIA), although more than 100% of T4 was recovered in the assays. RIA (double antibody-ANS system) seemed to be reliable for detection of T3 in milk, judging from recovery and dilution experiments. T3 was detectable in all samples obtained 1-4 months postpartum. The T3 concentration in milk was not correlated with protein concentration or daily volume. The concentration of T3 in milk was lower than that in serum and the mean ratio of serum T3 to milk T3 was 2.8 +/- 1.7 (mean +/- S.D.). No correlation was observed between the T3 concentration or daily T3 excretion in milk and the T3 concentration in serum. The total amount of T3 excreted in milk was estimated as only 5-1000 ng/day. The serum levels of thyrotropin, T4, free T4 and T3 were not significantly different between breast-fed and bottle-fed babies. These results indicate that T3 excretion in milk cannot be explained by simple diffusion from the blood into the mother's milk and that breast feeding has no influence on the pituitary thyroid axis of normal babies.